Resonant effects in dielectric gratings were identified in the early 1990's as having promising applications to free-space optical filtering and sensing. Resonant effects typically occur in sub-wavelength gratings, where the first-order diffracted mode corresponds not to freely propagating light but to a guided wave trapped in some dielectric layer. When a high-index-contrast grating is used, the guided waves are rapidly scattered and do not propagate very far laterally. As a result, the resonant feature can be considerably broadband and of high angular tolerance, which has been used to design novel types of highly reflective mirrors. Recently, sub-wavelength grating mirrors have been used to replace the top dielectric stacks in vertical-cavity surface-emitting lasers, and in novel micro-electromechanical devices. In addition to being more compact and relatively cheaper to fabricate, sub-wavelength grating mirrors also provide polarization control.
Although in recent years there have been a number of advances in sub-wavelength gratings, designers, manufacturers, and users of optical systems continue to seek grating enhancements that broaden the possible range of grating applications. For example, liquid crystal cells are conveniently controllable with voltage, but cannot be made sub-wavelength in the optical domain.